Similarity and Scaling in a Liquid-Fuel Ramjet Combustor

Liquid-fuel ramjets (LFRJs) exhibit high specific impulse (compared to rockets) due to ambient air intake for combustion and rely on storable liquid fuel at controllable mass flow rates. In this investigation, we perform a similarity analysis of an LFRJ combustor in order to determine parameters that can be applied to predict the behavior of an engine of any magnitude on the basis of test results obtained from engines of different scales. Similarity analysis accounting for geometry, transport phenomena, liquid-fuel dynamics, and chemistry is conducted. It defines a series of similarity rules resulting in pressure–diameter scaling. The scaling model was evaluated using Cantera chemical kinetics software and the Hybrid Chemistry Jet Propellant-8 liquid-fuel reaction mechanism, transport properties, and thermodynamic data. It simulates the combustion dynamics as those of a perfectly stirred reactor in order to determine the effects of the pressure and combustor size on combustion efficiency via the degree of reaction completion at various residence times. The simulation confirmed our scaling prediction that, for operating conditions where chemical kinetics are the main factor affecting combustion efficiency, we require pressures that are inversely proportional to the combustor dimensions.